Terephthalic acid (PTA), one of the raw materials for polyethylene terephthalate (PET) polymers, is currently produced by the oxidation of petroleum-derived para-xylene. Obtaining isomerically pure para-xylene (pX) from benzene-toluene-xylene (BTX) feeds requires several integrated processes such as disproportionation of toluene, isomerization of xylenes, and separation of para-xylene from an equilibrium mixture of xylenes. Oxidation of para-xylene to PTA is capital intensive, requiring both liquid phase oxidation in acetic acid and purification of crude terephthalic acid by selective hydrogenation. Furthermore, this process is petroleum based. There is a demand for PET from renewable sources, which may also have a cost advantage for the feedstock.
U.S. Pat. No. 7,385,081 (the '081 patent) describes the reaction of oxygenated derivatives of 2,5-dimethylfuran (DMF) to form terephthalic acid. However, that process did not utilize a catalyst, and, as a result, the yield obtained was extremely small. The '081 patent stated that the yield of terephthalic acid (and no other terephthalates) was 0.14 mol % using 2,5-furandicarboxylic acid (FDCA—CAS 3238-40-2) as the feed. Although the patent did not provide it, the yield of terephthalic acid using furan-2,5-dicarboxylate dimethyl ester (DM-FDCA—CAS 526-99-8) as the feed was calculated to be at most 0.023 mol % yield of terephthalic acid (and no other terephthalates) based on the data included in the patent. There are also several demonstrations in the prior art for conversion of DMF to para-xylene. Like U.S. Pat. No. 7,385,081, these processes utilize a cycloaddition pathway with ethylene acting as a dienophile adding to a furan as a diene, but in these cases the diene is DMF rather than an oxygenated derivative. These processes involve the activation of the diene (DMF) by electron-donating alkyl groups which leads the diene to become more reactive toward the dienophile compared to FDCA or DM-FDCA. However, the product is para-xylene rather than a terephthalate, and it must be further oxidized and purified in subsequent steps to obtain terephthalic acid. Furthermore, approaches that generate pX from biomass as a feedstock for PTA are not atom-efficient. Biomass (sugar) has a high oxygen content. Any route to PTA via pX must involve removing all of the oxygen from sugar, which generally requires hydrogen and yield losses. The oxygen is then added back in at considerable expense during pX oxidation. One example is the process described in US 2010/0331568 and U.S. Pat. No. 8,314,267 in which activated carbon, ZnCl2, rare-earth exchanged Y zeolite, silica gel, and gamma-alumina were used to catalyze the reaction of DMF and ethylene to form para-xylene. In WO 2009/110402, titanocene dichloride is used to catalyze the same reaction. In Do et al., “Elucidation of Diels-Alder Reaction Network of 2,5-Dimethyfuran and Ethylene on HY Zeolite Catalyst,” ACS Catal. 2013, 3, 41-46, HY zeolite is utilized, and the authors suggest a confinement effect of the faujusite cages as enhancing the Diels-Alder reaction (also using DMF as feed). In WO 2013/040514, Lewis acid catalysts such as copper triflate were utilized to obtain high conversion and selectivity for conversion of DMF to para-xylene. In Nikbin et al., A DFT study of the acid-catalyzed conversion of 2,5-dimethylfuran and ethylene to p-xylene,” J. Catal, 2013, 297, 35-43, using computational methods, the authors teach that for the reaction of DMF and ethylene to form para-xylene, the dehydration step is catalyzed by Brönsted acids but the cycloaddition step is catalyzed by Lewis acids. Supporting this, in Wang et al., “Selective Production of Aromatics from Alkylfurans over Solid Acid Catalysts,” Chem. Cat. Chem. 2013, 5, 2044-2050 the authors show that non-porous materials with both Lewis and Brönsted acidity such as WOx-ZrO2 and niobic acid have high activity and selectivity compared to materials that have only Lewis acidity. At 60% conversion of DMF, their work shows that WOx-ZrO2 is more than 3 times as active as H—Y zeolite, suggesting that Lewis acidity provides additional rate acceleration.
US 2014/0296600 describes a process for making para-xylene via cycloaddition of ethylene and DMF and subsequent dehydration using acidic heterogeneous catalysts and a solvent for DMF. The process is said to have high selectivity and high yields. The use of a solvent shows significant effects in the reduction of competing side reactions including hydrolysis of DMF to 2,5-hexanedione, alkylation of p-xylene, and polymerization of 2,5-hexanedione.
Pacheco and Davis, “Synthesis of terephthalic acid via Diels Alder reactions with ethylene and oxidized variants of 5-hydroxymethyfurfural,” PNAS, 111(23), p. 8363-8367 (2014), describe a process in which 5-hydroxymethylfurfural (HMF) is partially oxidized to 5-(hydroxymethyl) furoic acid (HMFA). The HMFA and the ether and ester derivatives of HMFA are reacted with ethylene in a Diels-Alder reaction to produce the desired aromatic product, which is then oxidized to PTA or dimethyl terephthalate. Dioxane is the preferred solvent. The process requires the oxidation of the reaction product of the Diels-Alder reaction. In addition, FDCA did not react.
Therefore, there is a need for processes for making terephthalic acid from non-petroleum feedstocks.